The present invention relates to a copying machine comprising a document illuminating apparatus including as its document exposure light source a fluorescent lamp which consumes less electric power and becomes less heated than a halogen lamp.
More particularly, the document illuminating apparatus includes, along with the fluorescent lamp, a lamp control device for the controlling power supply to the fluorescent lamp during a standby period such that the tube wall of the lamp is maintained at an optimal temperature for realizing a maximum emission efficiency. Since this apparatus employs a fluorescent lamp which requires a long warm-up time, it is desirable to maintain the tube wall at the optimal temperature during the standby period by utilizing the self-heating of the lamp in a controlled manner without seeking the aid of a heater or other separate heating means. Then a subsequent waiting time is shortened for starting a copying operation with an original document illuminated properly.
A known example of such a document illuminating apparatus is disclosed in Japanese Patent Publication No. 57-8471. This apparatus includes a temperature sensor for detecting the tube wall temperature of the fluorescent lamp. Power supply to the fluorescent lamp is controlled in response to detection results provided by the temperature sensor such that the lamp is turned off when the tube wall temperature exceeds the optimal temperature of about 40.degree. C. and is turned on when the tube wall temperature falls below the optimal temperature.
However, this document illuminating apparatus has the following disadvantage,
As noted above, the temperature sensor is used to detect the tube wall temperature of the fluorescent lamp for controlling the power supply to the lamp in order to maintain the tube wall temperature in the vicinity of 40.degree. C. This temperature detection is effected in a direct way and may therefore be reliable. In practice, however, it is difficult to detect the temperature of a tube wall portion having a direct influence on the document illumination while situating the temperature sensor out of interference with the document illumination. Thus the temperature must be detected at a wall position adjacent a tube end and, besides, the detection is vulnerable to the influence of ambient temperature. Consequently, the temperature sensor tends to provide inaccurate detection results. The control of power supply to the fluorescent lamp based on such detection results may cause the tube wall temperature of the fluorescent lamp to deviate to a serious degree from the optimal temperature.